The combustion of a solid rocket propellant charge creates a hostile environment characterized by extremely high temperature, pressure and turbulence in the interior of the rocket motor. Temperatures typically exceed 5,000.degree. F. (2760.degree. C.). (1.02.times.10.sup.5 g/cm.sup.2). Chamber gas velocity typically may reach Mach 0.2 (about 154 miles per hour or 67 meters per second at 20.degree. C.) at the aft end of the rocket motor. This environment is considered particularly hostile in a solid rocket motor because the combustion gas typically contains dispersed particles consisting essentially of aluminum oxide liquid droplets. These suspended droplets are believed to produce erosion of the rocket motor case insulation by abrading the interior of the rocket motor case by a sandblasting type effect. While the combustion of rocket propellant is usually of short duration, the conditions described above can destroy unprotected rocket motor casings prematurely and jeopardize the mission of the rocket.
Those parts of the rocket structure which are exposed to the high temperatures, pressures, and erosive flow conditions generated by the burning propellant are usually protected by applying a lining of insulation. Various materials, both filled and unfilled, have been tried as insulation. These include phenolic resins, epoxy resins, high temperature melamine-formaldehyde coatings, ceramics, polyester resins and the like. These materials, when cured, usually become rigid structures which crack or blister when exposed to the rapid temperature and pressure changes occurring when the propellant is burned. Some rubber-like elastomeric systems have been tried as insulation. These systems are ablative insulation because the elastomer is sacrificed or consumed during combustion, but nevertheless provides some protection for the rocket chamber. Such materials are capable of enduring in a rocket motor long enough to allow complete combustion of the solid propellant.
The best rocket insulation materials previously known to the art are elastomeric polymers reinforced with asbestos. Such systems are capable of enduring for a time sufficient to allow complete combustion of the propellant. Asbestos-reinforced elastomeric insulation systems are the subject of U.S. Pat. No. 3,421,970 to Daley et al., issued Jan. 14, 1969, and U.S. Pat. No. 3,347,047, to Hartz et al., issued Oct. 17, 1967. Those patents are hereby incorporated herein by reference.
Environmental and health concerns have led manufacturers to seek an acceptable replacement for the asbestos in rocket motor case insulation. One alternative elastomeric insulation contains aramid polymer fibers in combination with a powder filler. That insulation is disclosed in U.S. Pat. No. 4,492,779, and assigned to the owner of the present invention. Another alternative is elastomeric insulation which contains polybenzimidazole (PBI) polymer fibers in combination with the powder filler. That insulation is disclosed in U.S. Pat. No. 4,600,732 and assigned to the owner of the present invention. Both of said patents are incorporated herein by reference. The polyaramid fiber reinforced materials however, have been found less erosion resistant than asbestos reinforced materials.
It is therefore desirable that a means be found to increase the erosion resistance of fiber filled elastomeric insulation used in solid propellant rocket motors. It is also desirable that an improved means be provided to protect the interior of a solid propellant rocket motor during combustion of the propellant.